1. Field of the Invention
The invention relates to ceramic materials for thermal barriers and abradable coating systems in high temperature and high temperature cycling applications, and more particularly to ultra-pure zirconia materials for use in thermal barrier and abradable coating applications.
2. Description of the Related Art
Superior high-temperature properties are required to improve the performance of heat resistant and corrosion resistant members. These members include, for example gas turbine blades, combustor cans, ducting and nozzle guide vanes in combustion turbines and combined cycle power plants. Turbine blades are driven by hot gasses, and the efficiency of the gas turbine increases with the rise in operational temperature. The demand for continued improvement in efficiency has driven the system designers to specify increasingly higher turbine operating temperatures. Thus, there is a continuing need for materials that can achieve higher operational temperatures.
Thermal barrier coatings are used to insulate components, such as those in a gas turbine, operating at elevated temperatures. Thermal barriers allow increased operating temperature of gas turbines by protecting the coated part (or substrate) from direct exposure to the operating environment. An important consideration in the design of a thermal barrier is that the coating be a ceramic material having a crystalline structure containing beneficial cracks and voids, imparting strain tolerance. If there were no cracks in the coating, the thermal barrier would not function, because the differences in thermal expansion between the metal substrate system and the coating will cause interfacial stresses upon thermal cycling that are greater than the bond strength between them. By the creation of a crack network into the coating, a stress relief mechanism is introduced that allows the coating to survive numerous thermal cycles. Repeating crack networks are typically imparted into the coating on varying space scales by manipulating the thermodynamic and kinetic conditions of the manufacturing method, and different structures known to perform the coating task have been optimized likewise. In addition to this, cracks are also formed during service, so the structure formed upon coating manufacture changes with time, depending on the starting material phases in the manufactured coating and thermal conditions during service.
Another design factor determining coating lifetime is the sintering rate of the coating. When the coating is cycled above half of its absolute melting temperature, the coating begins to sinter causing volume shrinkage. As the coating shrinks, the stress difference between the coating and substrate increases. At a certain amount of shrinkage (which varies depending on the type of structure and thermal conditions during service), the stress difference exceeds the bonding strength of the coating and it becomes detached. Decreasing the sintering rate of the thermal barrier increases the amount of time before the catastrophic shrinkage is experienced, so it can become a major design consideration. For high purity zirconia alloys, the onset of sintering commences at temperatures above 1000° C.
Historically, high temperature thermal barrier coatings have been based on alloys of zirconia. Hafnia may also be employed due to its chemical similarity to zirconia, but is generally cost-prohibitive. Hafnia also is typically present in most zirconia materials in more than trace amounts due to difficulty in separating the two oxides. Zirconia and/or hafnia have the following combination of desirable properties that other known ceramic systems do not possesses for the application. First, zirconia alloys have some of the highest melting points of all ceramics, and this means theoretically some of the highest temperatures for which the onset of sintering occurs. Second, zirconia alloys have one of the lowest thermal conductivities of all ceramics. Third, zirconia has one of the highest coefficients of thermal expansion of all ceramics, so it is most compatible with transition metal alloys during thermal cycling.
Zirconia alone cannot fulfill the coating requirements because it undergoes a phase transformation from tetragonal to monoclinic during thermal cycling. This transformation is presumed to cause a detrimental volume change resulting in large strain differences between the coating and the substrate. When the resulting stresses exceed the bond strength of the coating to the substrate, the coating will detach. For this reason a phase stabilizer is added to the zirconia and/or hafnia, such as yttria, which suppresses the tetragonal to monoclinic phase transformation.
Thermal spray abradable coatings are commonly used in gas turbine applications. Abradable coatings are designed to preferentially abrade when contact is made with a mating part. These coatings have low structural integrity so they are readily abraded when they come into contact with a moving surface with higher structural integrity (such as the blade of a turbine). The coatings are designed so as not to damage the mating surface. In many applications abradable coatings are subject to the same thermal cycling conditions as the thermal barriers described above. Thus, there is a continuing need for materials suitable for abradable coatings that can achieve higher operational temperatures.
Some previous efforts to improve coating life have focused on the coating material and microstructure upon entry into service. However, the heat cycle of in service parts also causes cracks throughout the service life of the part. Thus, the microstructure formed upon coating manufacture changes with time, depending on the starting material phases in the manufactured coating and thermal conditions during service. Because a consistent optimal crack network is not typically maintainable throughout the service life of the part, coating lifetime is ultimately determined by the material selection and its manufacturing process. There remains a need in the art for a coating material, coating material manufacturing method, and coating manufacturing method that address the changes in the coating microstructure during its service lifetime.